The present invention relates to a heat-exchanger particularly useful for low temperature applications, and also to a method and to apparatus for making such a heat-exchanger. The invention is especially useful in dilution refrigerators based upon the "evaporation" of a helium liquid into a Fermi "gas" for obtaining temperatures approaching absolute zero, and is therefore described below in connection with such an application.
The dilution refrigerator is now a standard research tool used in low temperature laboratories to cool samples continuously below 10 mK (0.010 K) near absolute zero in temperature. The success of the dilution refrigerator depends upon the use of highly efficient heat-exchangers in which the warm incoming .sup.3 He liquid exchanges heat with the cold exiting gas. At these very low temperatures, a thermal boundary resistance, known as Kapitza boundary resistance, introduces a barrier to heat transfer. This resistance increases as T.sup.-2 increases to T.sup.-3 and becomes exceptionally high below 10 mK. To overcome this boundary resistance between the liquids and metal body of the heat-exchanger, large surface contact areas on the order of 10 m.sup.2 to 100 m.sup.2 are required.
Two types of heat-exchangers, namely the continuous type and the step-type, are now used in cooling samples near absolute zero. The continuous-type heat-exchanger inherently is more efficient and theoretically can produce a lower temperature since it provides a temperature gradient between the inlet and outlet ends of each flow path, e.g., by using low thermal-conductivity materials, such as stainless steel and cupro-nickel alloys. The step-type heat-exchanger does not provide a significant temperature gradient between the inlet and outlet ends of each flow path, and is therefore less efficient, but is frequently much simpler and less expensive to produce.
Until 1978, step-type heat-exchangers has been fabricated from coarse sintered powder copper sponges or fine copper wires; typical contact areas were 0.1 m.sup.2. Recently, ultra-fine-diameter silver powder having 700 .ANG. mean diameter, and copper powder having 500 .ANG. mean diameter, have become available (.ANG.=10.sup.-8 cm). In 1978, this fine silver powder was successfully sintered to a thin cupro nickel foil to form a continuous-type heat-exchanger, commonly called a Frossati heat-exchanger, and it was demonstrated that temperatures of 2 mK could be achieved continuously in the dilution refrigerator using six of these heat-exchangers. This is probably the lowest temperature yet reached by a dilution refrigerator. The contact surface area for each liquid in the Frossati heat-exchanger is about 120 m.sup.2, which is about 1000 times greater than the contact area in the earlier step-type heat-exchangers. Several commercial companies have started to manufacture the Frossati the heat-exchanger, but a number of problems have arisen. Thus, the sintering of the very fine powder to both sides of the foil is a complicated procedure requiring that a high pressure, of about 500 Kg/cm.sup.2, be applied to the powder and foil at 200.degree. C. in a hydrogen atmosphere. Also, sealing the foil on both sides to confine the liquids within the heat exchanger requires welding a cover to each side of the foil, but the welded joints have been found to be subject to leaks after cycling between room temperature and liquid helium temperatures. Moreover it is very difficult, if possible at all, to thermally anchor electrical wires, experimental feed lines, and a 50 mK thermal shield to the above foil-type heat-exchangers.